Association of anti‐factor Xa‐guided anticoagulation with hemorrhage during ECMO support: A systematic review and meta‐analysis

Abstract Background The use of extracorporeal membrane oxygenation (ECMO) is associated with complex hemostatic changes. Systemic anticoagulation is initiated to prevent clotting in the ECMO system, but this comes with an increased risk of bleeding. Evidence on the use of anti‐Xa‐guided monitoring to prevent bleeding during ECMO support is limited. Therefore, we aimed to analyze the association between anti‐factor Xa‐guided anticoagulation and hemorrhage during ECMO. Methods A systematic review and meta‐analysis was performed (up to August 2023). PROSPERO: CRD42023448888. Results Twenty‐six studies comprising 2293 patients were included in the analysis, with six works being part of the meta‐analysis. The mean anti‐Xa values did not show a significant difference between patients with and without hemorrhage (standardized mean difference −0.05; 95% confidence interval [CI]: −0.19; 0.28, p = .69). We found a positive correlation between anti‐Xa levels and unfractionated heparin dose (UFH; pooled estimate of correlation coefficients 0.44; 95% CI: 0.33; 0.55, p < .001). The most frequent complications were any type of hemorrhage (pooled 36%) and thrombosis (33%). Nearly half of the critically ill patients did not survive to hospital discharge (47%). Conclusions The most appropriate tool for anticoagulation monitoring in ECMO patients is uncertain. Our analysis did not reveal a significant difference in anti‐Xa levels in patients with and without hemorrhagic events. However, we found a moderate correlation between anti‐Xa and the UFH dose, supporting its utilization in monitoring UFH anticoagulation. Given the limitations of time‐guided monitoring methods, the role of anti‐Xa is promising and further research is warranted.

Its anticoagulant properties are attributed to its ability to enhance the activity of antithrombin, downregulating thrombin and factor Xa. 3,12 Alongside UFH, heparin-coated circuits are employed with the goal of minimizing the requirements for systemic anticoagulation. 13The primary mechanism of action involves heparin binding to the circuit surfaces, mimicking the antithrombogenic effects of heparan on the endothelium.These surfaces release minimal amounts of heparin into the blood, resulting in negligible systemic effects. 13Due to the inherent limitations of UFH, systemic anticoagulation is increasingly achieved using alternative methods, such as direct thrombin inhibitors (e.g., argatroban, bivalirudin, etc.), direct and indirect factor Xa inhibitors (e.g., rivaroxaban, apixaban, and edoxaban), heparinoids (e.g., danaparoid), and other novel anticoagulants (e.g., factor XIIa inhibitors, circuit releasing compounds, nitric oxide, etc.). 3 The Extracorporeal Life Support Organization (ELSO) recommends the use of time-guided anticoagulation tools (i.e., activated clotting time [ACT], activated prothrombin time [aPTT], clotting times generated from viscoelastic testing), or anti-factor Xa assays for monitoring anticoagulation with UFH. 4,14,15Depending on the patient's hemostatic capacity, ELSO advocates anticoagulation goals of 1.5 to 2.5 times the patient's baseline for aPTT and 0.3-0.7 IU/mL for anti-factor Xa assays. 14The International Society on Thrombosis and Haemostasis (ISTH) recommends anticoagulation using UFH, primarily monitored with anti-factor Xa, with target levels of 0.3-0.5 IU/mL.Alternatively, ISTH suggests achieving an ACT of 180-220 s or an aPTT of 50-70 s. 16 However, these anticoagulation goals have not been validated in controlled randomized trials or in critically ill patients undergoing ECMO.The most appropriate UFH monitoring tool in ECMO patients remains unclear, and strong evidence is lacking.Systematized evidence on the association of anti-Xa levels with hemorrhagic events in patients receiving ECMO does not exist.Therefore, we aimed to provide a comprehensive review and meta-analysis investigating the relationship between measured antifactor Xa levels and hemorrhage, including its correlation with the UFH dose.

| MATERIAL AND METHODS
A systematic literature review and meta-analysis were conducted on studies investigating anticoagulation monitoring using anti-factor Xa.
The study protocol is registered in the international database of prospectively registered systematic reviews PROSPERO database-CRD42023448888, and the research was carried out in accordance with the PRISMA guidelines (Supporting Information S1: Table S1). 17e primary objective was to systematize evidence concerning the association between anti-Xa levels and hemorrhage during ECMO support.Secondary endpoints included assessing the correlation between anti-factor Xa levels and the UFH infusion rate, as well as the incidence of adverse events.All selected works comprised patients requiring ECMO, using anti-factor Xa monitoring, and reporting the incidence of bleeding (Supporting Information S1: Table S2).

| Search strategy
We performed a systematic search of literature in the PubMed and Scopus databases (up to August 2023).The search strategy involved combining terms related to ECMO support and anticoagulation monitoring (Supporting Information S1: Table S3).
Ensuring the comprehensiveness of our search, we also reviewed the references of all included works.In case where full-text works or detailed information on anti-Xa levels were unavailable, authors of original work were contacted.Our inclusion criteria encompassed publications reporting on ECMO support, anti-Xa anticoagulation monitoring, and bleeding.We excluded duplicate publications, articles reporting results from the same center, and systematic reviews or meta-analyses.All data, search, and study restrictions are outlined in Supporting Information S1: Table S2.Two authors conducted screening of the literature (D.J., S. R.).

| Data extraction
A summary of the extracted data is reported in Supporting Information S1: Table S4.To standardize the results of the analyzed works and facilitate comparison, the following calculations were performed: (A) conversion of percentages into original values, (B) the portion of male or female patients was calculated from the available information, (C) summation of outcomes of interest if provided for only one group, and (D) conversion of provided median values with ranges into means and standard deviations. 18All calculations were performed independently by two authors (D.J., S. R.).

| Quality assessment of included studies
The methodological quality was assessed using the Newcastle-Ottawa scale (NOS). 19Works scoring more than seven NOS stars were considered to be of good quality, those with at least five stars were considered fair, and those with fewer than five stars as lowquality (Table 1 ).Standardized mean differences (SMD) were utilized to compare patient groups, and these were calculated by pooling individual publication data using random or fixed effect models.We used the inverse variance methods with Fisher's z transformation for the pooled estimate of correlation coefficients.For ease of interpretation, back transformation (z to r transformation) to the original coefficients level was performed.
Heterogeneity was analyzed by τ 2 and Cochran's Q test, and the results were reported with I 2 .Using funnel plot and Egger's test, we assessed the publication bias.A significance level of 0.05 was applied.

| Search results and included studies
The initial search yielded 1419 and 1758 publications in PubMed and Scopus, respectively.Following the removal of duplicates, the titles and abstracts of 2213 works were screened.In the next step, 2130 works were excluded: 1175 due to irrelevant outcomes, 535 based on publication type, and 377 addressed irrelevant population (Figure 1).
The excluded articles, along with the reason for exclusion, are detailed in Supporting Information S1: Table S5.Thereby, 117 articles underwent full-text screening, resulting in the exclusion of an additional 91 articles.Finally, 26 studies met the inclusion criteria, with six studies reporting on the anti-Xa levels being included in the meta-analysis.
The characteristics of the included articles are available in Table 1.The analyzed publications represent the situation from the USA (n = 18), France (n = 3), and one each from the Netherlands, Germany, Vietnam, Saudi Arabia, and Australia.Twenty studies reported on both venovenous (VV) and venoarterial (VA) ECMO support, while four focused solely on VA-ECMO.Additionally, 18 authors provided information on the correlation between UFH infusion and anti-Xa blood levels.primary anticoagulant, with target anti-Xa levels ranging from 0.2 to 1.0 IU/mL, Table 1 and Supporting Information S1: Table S6.
Information on cerebral hemorrhage was reported in 11 studies, with a pooled rate of 10% (95% CI: 6.3; 14.9), while pulmonary and gastrointestinal bleeding were reported with pooled rate of 5% and 4%, respectively.2 and Supporting Information S1: Table S7).

| Hemorrhage and anti-Xa
The majority of the included works intended to describe the association between anti-Xa and hemorrhage, as well as the correlation between anti-Xa levels and the UFH dose.Seven studies (n = 391) reported the average anti-Xa levels for patients with and without bleeding (Table 1).The meta-analysis (six studies, n = 352) revealed no significant difference in the average values of anti-Xa between patients with and without bleeding events (SMD = 0.05; 95% CI: −0.19; 0.28, p = .69);with heterogeneity of I 2 = 37% (95% CI: 0.0; 74.9, Q = 7.9, τ 2 = 0.06, p = .16)(Figure 2).One work did not report the variance of measured anti-Xa levels and could not be included in the analysis. 20Finally, we did not identify the presence of publication bias (p = .160)(Supporting Information S1: Figure S1).

| Sensitivity analysis
The sensitivity analysis identified Descamps et al. as a study that could significantly contribute to the heterogeneity observed.The results of the sensitivity analysis were consistent with the main analysis findings (Supporting Information S1: Figures S1.1-S1.4).
In the subgroup analysis based on pediatric and adult patient populations, the meta-analysis of studies focusing on adult patients

| Correlation of UFH dose with anti-Xa
Eighteen studies (n = 878) provided data on the correlation between UFH infusion and anti-Xa levels.Out of these, fourteen articles T A B L E 2 ECMO-related outcomes and adverse events (n = 26).(n = 584) were eligible for inclusion in the meta-analysis of correlation coefficients (Figure 3 and Supporting Information S1: Table S6).The correlation coefficients reported in the analyzed studies ranged from 0.1 to 0.75, all indicating a positive correlation between UFH infusion and anti-Xa levels.The pooled estimate of correlation coefficients was 0.44 (95% CI: 0.33; 0.55, p < .001),with heterogeneity of I 2 = 62% (τ 2 = 0.02, p = .001)(Figure 3).Sensitivity analysis (excluding Yabordi et al.) reaffirmed the findings of the main analysis (Supporting Information S1: Figures S4-S5 and Table S8).
In the subgroup analysis based on pediatric and adult patient populations, four studies (n = 212) reported on the correlation of UFH infusion with anti-Xa levels for adult patients and 10 (n = 372) for pediatric population.For adult patients, the pooled estimate of correlation coefficients was 0.35 (95% CI: 0.16; 0.52, p < .001),with heterogeneity of I 2 = 40% (τ 2 = 0.02, p = .17

| DISCUSSION
The present work evaluated use of anti-Xa-guided anticoagulation monitoring in critically ill patients requiring ECMO support.Our analysis encompassed 26 articles with 2293 patients, representing the most extensive analysis to date of anti-Xa-guided anticoagulation monitoring.There was no significant difference in anti-Xa levels between patients with and without hemorrhagic events.However, for the first time we report systematized evidence on a moderate correlation between UFH dose and anti-Xa levels in ECMOsupported patients.
F I G U R E 2 Forest plot: Average anti-Xa values among patients with and without hemorrhagic event.CI, confidence interval; ECMO, extracorporeal membrane oxygenation; SMD, standardized mean differences.
F I G U R E 3 Forest plot: Meta-analysis of correlation coefficients.CI, confidence interval; COR, correlation coefficient; ECMO, extracorporeal membrane oxygenation.

| Anticoagulation monitoring during ECMO support
The optimal anticoagulation in critically ill is challenging, especially in those needing ECMO support.Balancing anticoagulation to avoid thromboembolic complications against hemorrhage in this patient population remains difficult.2][23] The most recent edition of the ELSO Red Book highlighted the complexity of anticoagulation and its monitoring, acknowledging the scarcity of evidence and providing limited recommendations in this regard. 24e current anticoagulation goals lack validation in ECMO patients and are derived from a prospective conducted in adults back in 1972. 3,15,25According to the ISTH, the recommended antifactor Xa anticoagulation targets (0.3-0.5 U/mL) are slightly lower than those of ELSO (0.3-0.7 U/mL). 16These recommendations are supported by findings from three retrospective and two randomized controlled trials.Authors of mentioned works have reported an increased rate of major hemorrhage and hemorrhage-related death among patients with higher anticoagulation targets (180-220 vs. 140-160 s), with no significant disparity observed in the frequency of oxygenator replacements or major thrombotic events. 26Another trial found no discernible difference in outcomes when comparing lowand high-dose UFH anticoagulation based on ACT, 27 or when comparing restricted or conventional anti-factor Xa protocols (0.3-0.7 IU/mL vs. 0.2-0.5 IU/mL). 28However, all analyzed studies are constrained by small patient populations, indicating the need for further research.
However, the altered hemostatic capacity of ECMO patients, particularly in case of hyperinflammation, may limit the UFH effect by increasing its binding to acute phase proteins. 3In such circumstances, the use of time-guided monitoring strategies may be biased with the limitations of the available tests.Optimal use of aPTT requires a linear relationship between aPTT and UFH dose, which is not the case in severely ill, where aPTT could be altered by the existing hemostatic changes (i.e., coagulopathy-associated consumption of coagulation factors, elevated C-reactive protein, lupus inhibitor, fibrinogen, factor VIII, etc.).0][31] Finally, the poor correlation between timebased methods (both aPTT and ACT) and UFH dose reduces the applicability of these assays, despite their wide availability, low cost, and short turnaround time. 3,32Thus, the introduction of alternative methods or multimodal approach is becoming more interesting for clinicians, but the evidence from large trials is still missing. 3,15,23 the analyzed articles, we observed different anticoagulation targets, ranging from 0.2 to 1.0 for anti-Xa, 45 to 100, and 160 to 225 s for aPTT and ACT, respectively.Interestingly, there was no association between higher or lower anticoagulation targets and factors such as patient population, indication for ECMO support, or type of support.This finding is surprising considering that coagulation processes may differ between pediatric and adult population; or between patients with acute respiratory distress syndrome and those with cardiogenic shock.However, this may be attributed to the current diversity in protocols among different centers and the lack of standardization.
Our meta-analysis did not reveal a significant difference in anti-Xa values between ECMO patients with and without hemorrhage.
This could be attributed to the limited evidence available on anti-Xaguided anticoagulation monitoring, or the quality of reporting.Among the 26 studies included, only seven authors reported anti-Xa values for patients with and without bleeding.Additionally, in four studies, anti-Xa levels were slightly higher in patients without hemorrhage compared with those with hemorrhage.This could suggest that clinicians may adopt a more cautious approach to anticoagulation in patients who are clinically identified as having an increased risk of hemorrhage, potentially leading to contradictory findings.Clinicians might strive for lower anti-Xa goals to mitigate bleeding risk while still maintaining anticoagulation within the predefined range.This might be particularly relevant in the pediatric population, as all studies reported lower average anti-Xa levels in patients with bleeding.
Conversely, only two studies provided average anti-Xa levels for adult patients, both of which reported higher average anti-Xa values in patients experiencing hemorrhage events.
An additional factor that may explain our findings across all studies is the variability in the timing of anti-Xa measurement, which is often not clearly specified or standardized in retrospective studies.
Anti-Xa levels can be reported as an average over the whole ECMO period, as the highest/lowest value shortly before the event, or even more complex in case of multiple bleeding events.The only way to overcome this key limitation are prospective or randomized trials, where the time point of adverse events and laboratory measurements could be exactly examined.
Moreover, it's important to consider the intrinsic limitations of the anti-Xa assay, which could act as a confounding factor in the contradictory findings observed.Factors such as high plasma-free hemoglobin, hyperbilirubinemia, or hyperlipidaemia can lead to falsely low anti-Xa values.Both hyperbilirubinemia and increased free hemoglobin (due to hemolysis) are frequently seen in patients receiving ECMO, potentially limiting its application.Moreover, the anti-Xa is a plasma-based assay, excluding the role of platelet and fibrinogen in the formation of clot.Therefore, underestimation of the platelets/fibrinogen role or false low anti-Xa levels may lead to increased UFH dose and unintentional over-anticoagulation. 14,24,33 Despite these limitations, anti-Xa may still offer advantages over traditional assays like aPTT, particularly in situations involving elevated C-reactive protein or the presence of lupus anticoagulant.

| Correlation of UFH dose with anti-Xa
Fourteen articles were included in the meta-analysis of correlation coefficients, all of which demonstrated a positive correlation between anti-Xa and the UFH dose.Despite existing heterogeneity, events.However, we found a moderate correlation between anti-Xa and the UFH dose, supporting its utilization in monitoring UFH anticoagulation.Given the limitations of time-guided monitoring methods, the role of anti-Xa is promising, and further research is warranted.
Between 2004 and 2020, a total of 2293 patients requiring ECMO support were included in the study.All authors used UFH as the F I G U R E 1 PRISMA flowchart.PRISMA, Preferred Reporting Items for Systematic Reviews and Meta-analyses.